Study on the convection and dispersion behavior of hexavalent chromium in cement-attapulgite cutoff wall

Industrialization has elevated hexavalent chromium (Cr(VI)) contamination in soil and groundwater. While vertical cutoff walls are common remediation tools, the influence of hydraulic gradients on contaminant transport within these barriers remains inadequately studied. This investigation systematically examines Cr(VI) migration through cement-attapulgite cutoff walls under varying hydraulic gradients (10-40) via laboratory experiments. Results demonstrate that molecular diffusion consistently dominates over mechanical dispersion in Cr(VI) transport across this gradient range. Although mechanical dispersion and convection intensify with increasing gradients, adsorption persists as the primary migration mechanism. Analysis incorporating diffusion reveals that dispersivity is independent of seepage velocity, whereas the hydrodynamic dispersion coefficient increases and the retardation factor decreases with rising velocity. Based on these relationships, a predictive model is proposed for estimating the Cr(VI) retardation factor under different seepage velocities. Furthermore, a Cr(VI) migration model utilizing Hydrus software was developed to evaluate wall performance under diverse hydraulic heads. These findings elucidate fundamental transport mechanisms and provide critical predictive tools, enhancing the understanding and application of cutoff wall technology for Cr(VI) containment.